There are 6 basic types of lenses. These are:

a) Double Convex
b) Plano Convex
c) Convex Meniscus
d) Double Concave
e) Plano Concave
f) You guessed it- Concave Meniscus



A lens bends light. If you have glasses, then you are looking through a lens. A Double Convex lens converges the light while a Double Concave lens causes light to diverge. Like a mirror, a lens has a focal length and a focus. The focal length (measured from the focus to the center) is the same on either side of the lens.

	Click on this logo to see how a lens converges light.
	Click on this logo to see how a lens diverges light.

The curved mirror equation (see Mirrors) is also known as the Lens Makers Equation- it can be applied to lenses as well as mirrors, in exactly the same fashion.

For a very long time, there were two theories on light. The first theory, held and developed by Isaac Newton, was the Particle Theory. The Particle Theory stated that light consisted of a stream of particles (called corpuscles) that shoot out like bullets from a light source.

The second theory dealt with waves. The Wave Theory held that light is a wave and therefore all the rules of waves apply.

Both theories had to explain why light did what it did. Neither theory had difficulty explaining reflection. However, on the topic of refraction, the particle theory ran into a little bit of difficulty. Newton (a.k.a. the apple guy), who was the main supporter of the particle theory, explained that when light entered water from air, the water molecules attracted the corpuscles which caused them to bend. The force of attraction also caused the light to speed up in the water. Now, we Phlying Physicists have a few problems with Newton's explanation. One of them would be, why would the particles be attracted only one way? If the water molecules are spread throughout the entire medium, which they are, how could one say that all of the particles would be bent the same direction? They should bend in different directions, and the light should not refract at all. However, seeing as how Newton was the prominent scientist in his day, no one questioned him with much success. The lesson there: If you're famous, you can get away with anything. Remember that, it will be on the test at the end of this unit. It was not until the 1850's that Newton was proven absolutely wrong. Diffraction also presented a problem for the particle theorists. Diffraction is the bending of light around a corner or through a gap. Newton said that because light was a particle, it did not bend around a corner. Wave theorists tried to show that light did bend around corners. However, since technology at that time was not very advanced, they were unable to show this until 1805. Once diffraction of light was proven to exist, the wave theory was accepted.

As we mentioned, diffraction is the bending of light. No, you can't bend light with a pair or pliers. Light bends itself! Look below for some beautifully drawn examples of how light bends around a corner, and through a gap.




Proving Diffraction of light was of huge significance to physicists for a long, long time. In 1800, Jean Fresnel determined that the amount of diffraction was related to the wavelength and the size of the gap:


Amount of Diffraction = Wavelength/Gap Size

	Click here to see how diffraction works. Blue and Violet light have very small wavelengths, and red light has a very large wavelength. To change the amount of diffraction, adjust the wavelength and the gap size. (Just like in Jean Fresnel's equation!)

To see diffraction, a scientist named Young did an experiment with a very small gap. We hope you read the previous lesson on wave interference- because now it becomes significant. If you didn't read them - GO BACK! DO YOU THINK WE'RE TEACHING THIS STUFF BECAUSE ITS USELESS?! Young's experiment consisted of placing a double slit in front of a light source. When the light waves entered through the two slits and diffracted, the crests and troughs of each wave would meet each other and combine to form either Nodal Points or Anti-Nodal Points.

If light is a wave, we should get toggling bright and dark patches on the screen or surface in which the light is being projected. This is because when the various crests and troughs combine, crests will create anti-nodes, or bright spots, and troughs will create nodes, or dark spots. From 1802 to 1804, Thomas Young performed a series of experiments using a double slit to try and get this pattern; and he did. It took 11 years to convince people of his findings.

His experiment formulated Young's Wavelength Equation:


wavelength = x * d / l

x = the distance between nodal lines. d = the distance between slit centers. l = the distance between the slits and the screen.

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Example Problem

A double slit apparatus has the slit center 0.26 mm apart and the screen 2 m away. 40 dark lines have a total width of 20 cm. Find the light's wavelength and frequency.

Given:

x = 40 segments in 20 cm,

width is therefore 0.5 cm --> 0.005 m

d = 0.26 mm ---> 0.00026 m

l = 2m

Formula:

wavelength = xd/l

Universal Wave Equation:

velocity of wave = wavelength * frequency

Work:

a) Find the wavelength

wavelength = (0.005 m) * (0.00026 m) / (2m)

wavelength = 6.5 * 10-7 m

b) Find the frequency:

velocity of wave

(C = 3.0 * 108 m/s) = (6.5 * 10-7 m) * f

4.62 * 1014 Hz = f

Answer:

The wavelength is equal to 6.5 * 10-7 m and the frequency is

4.62 * 1014 Hz.

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Primary colors are the colors from which we can make any other visible color, usually by mixing levels of each primary together. The primary colors of light are Red, Green and Blue. If you have taken art class, they would have probably told you that the primary colors are Red, Yellow and Blue. But before you go saying things to your art teacher that would have gotten "your mouth washed out with soap" (in your younger years), you should know that those are referred to as the primary colors of pigments and they are often used in day to day life, but not in physics class. Here's a neat memory trick- "Yellow Yucky, Green Groovy." If you remember that, you'll never get them mixed up. We hope.

Light differs in wavelength and frequency. Red light has the greatest wavelength measuring from 610 nm to 750 nm. Violet light has the smallest wavelength measuring from 400 nm to 450 nm. Frequency is the opposite- Violet light has a frequency from 7.5 * 1014 Hz -->6.67 * 1014 Hz. Red light measures from 4.92 * 1014 Hz to 4.0 * 1014 Hz.

An object only reflects its own color of light. A red object reflects red light. The color of the surface is reflected and the others absorbed. Black is the absence of color and does not reflect any color of light. If you shine green light on a green object, it will appear black. Why? This is because there is no green light to reflect, only red, which is absorbed. White light is the presence of all colors of light, so shining white light on something doesn't change its appearance. Here's an experiment you can do at home- you'll need a partner, and a flashlight. Shine the flashlight into your partners eyes, at close range. Your results should be definitive- they will get annoyed at you, and their eyes wont appear to change color. (It's WHITE light!) Actually, your shouldn't shine a flashlight into anyone's eyes- it could be harmful.



If you exposed this flag to red light, the red would stay red and the white would turn red...an all red flag.

You thought it was paint, didn't you? Nope. When light hits a particle, it diffracts or scatters. The relationship is


S(Scatter) = 1/wavelength E4

Blue and Violet light have the smallest wavelengths therefore they scatter the most. Thus, there is more scattered blue light than any other color- and the sky appears to be blue. The size of the particles in the air also determine the color of the sky. You will have red skies with dust or smoke.